x86: Remove superfluous NULL pointer check in destroy_irq()

This takes care of the following entry from Dan's list:

  arch/x86/kernel/apic/io_apic.c +3241 destroy_irq(11) warning: variable derefenced before check 'desc'

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Eugene Teo <eteo@redhat.com>
Cc: Julia Lawall <julia@diku.dk>
LKML-Reference: <200907302321.19086.bzolnier@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>